This invention relates to toroidal transformers and more particularly to an interwinding shield for minimizing the electrostatic coupling between primary and secondary windings of toroidal transformers.
Toroidal magnetic cores are frequently used in electronics because of their compact size, tight coupling and low flux leakage characteristics. Because of the close proximity between winding layers on the core required to achieve tight coupling, capacitance between the windings may be high. High interwinding capacitance may cause severe noise problems, particularly when the transformer is used to couple circuits with very steep voltage changes such as found in power inverters.
A common means of reducing the effect of winding capacitance is to place an electrostatic shield between windings to shunt capacitive coupled currents away from sensitive circuits. With normal cut or laminated cores with cylindrical openings, the shield may consist of a single layer of foil separated by two windings. Shielding a toroidal transformer is more difficult because of its shape. Since the electromagnetic induction between transformer windings must not be adversely effected by an electrostatic shield, the shield cannot constitute a short-circuited turn around a transformer winding. Known forms of toroidal transformer electrostatic shields include: a single close wound layer or wire with the start and finish of the winding open circuited, and a shield comprising two layers of metallic paint being separated by a narrow strip of insulating tape. The wound shield is subject to gaps between the turns which lessens its effectiveness while the painted shield adds complexity to the transformer assembly process. The present invention seeks to provide a toroidal transformer with a simple yet effective electrostatic shield between the windings.